In a typical application a spot of light from an LED
is incident upon a quad cell. The position information is contained
in the amplitude of the four signals from the quad cell. The processor
forces the sum of these four signals to equal a reference voltage,
which compensates for ageing of the LED and responsitivity of the
quad cell.
The position information is the difference between the signals of
opposite pairs of cells ( a+b-(c+d)) where a,b,c and d are the four
quad cell signals. This difference signal is scaled to provide a ±
10 v output coresponding to the total range of motion (typically 1
mm) from the sensor.

Controller:

The PID controller uses a 2nd order rate loop (gain
and zero placement) enclosed within the position loop. Ki and Kd control
the zero location and gain. Kp sets the position loop gain. It receives
the position information from the sensor processor , and an external
command position input. It generates the motor driver command signal
which forces the sensor position to match the command position input.

Driver:

The power driver is a PWM driver operating at 100kHz.
The output is filtered at ~15 kHz to remove most of the fundamental
and almost all of the higher harmonics of the PWM frequency. The output
is almost a pure replica of the command input with very little noise.
The power rail is typically 24 volts, however rails up to 100 volts
can be accomodated.